Monday, September 13, 2010

RFID made simple

More and more people are using the Sceptre ARM7 board from Elektor, which is good. The Sceptre is supported with an open source library in C and I am currently working on adding new functionality to it. As I wrote in a previous article I spent quite some time understanding the BTM222/112 Bluetooth modules and this will soon be included in the library.

I also received a request for RFID support and so I implemented a driver for the EM4095 transceiver (EM Microelectronic) that can read the tags you can buy for cheap on eBay (for instance).

To get me started I bought in Hong-Kong (through eBay of course) for 17.50 euros (shipping included!) a fully functional RFID proximity door entry access system with 10 tags and 5 badges. Only a week later the kit arrived.

This turned out to be a pretty nice product actually; its only downside is its cheap plastic housing that will probably not survive very long outdoors. Rain and intruders will not have a very hard time getting to the electronics.

Top side of PCB

Bottom side of PCB

The antenna is slightly larger than the PCB

Interesting about this unit is that it uses a "discrete" (well, sort of, compared to a single chip solution) radio, built around a 4069 and a 74393. I traced the circuit (see below), but I didn’t measure the capacitors (all SMD) except for C3 which has 2.2 nF (222J) printed on it. It seems hard to believe that this solution is cheaper than using a dedicated chip like the EM4095, but it probably is. Or maybe the design is older?

The processor is a W78E052DDG, a pretty standard 8051-clone by Nuvoton (formerly known as Winbond).

I only used the antenna from the kit (and of course the tags) for my experiments since I had the EM4095 chip on a break-out board and now that I am done, I am looking for an application for my "RFID proximity door entry access system". What about a cat door opener? Hmmm…

P.S. The RFID driver will be included in the Sceptre library. An update is scheduled with the December issue of Elektor (available half of Novembre). If you can't wait that long, just drop me a line.